An MIMO (Multiple Input Multiple Output) transmission scheme in which plural antennas are used between a base station and a user equipment terminal (which is typically a mobile station and may be a fixed station) is used for communications. According to the MIMO transmission scheme, plural streams which are formed by replicating a signal stream to be transmitted, for example, are multiplied by weighting factors, and then directional beams are generated. By using the directional beams, quality of transmission signals or a transmission speed can be improved. The weighting factor used for the MIMO transmission scheme is called a precoding vector or a precoding matrix.
In an E-UTRA (Evolved UMTS Terrestrial Radio Access) system and an LTE (Long Term Evolution) system, which are standardized in 3GPP (3rd Generation Partnership Project), an MIMO transmission scheme with four transmission antennas at the maximum in downlink is used. FIG. 1 shows an arrangement of reference signals in the LTE system (see non-patent document 1). The reference signal may be defined as a predetermined bit sequence used by a receiver to receive and demodulate other symbols. Alternatively, the reference signal may be simply defined as a reference signal known to both a transmitter and a receiver. Specifically, as shown in FIG. 1, reference signals to be used for channel estimation of transmission signals from the first and second antennas are multiplexed into first, fifth, eighth, and twelfth OFDM symbols. In addition, reference signals to be used for channel estimation of transmission signals from the third and fourth antennas are multiplexed into second and ninth OFDM symbols.
[Non-patent document 1] 3GPP, TS36.211 (V8.1.0), “Physical channels and modulation (Release8),” Nov. 2007.